Pneumatic impact ground piercing tools have been commercially useful products for decades. Self-propelled pneumatic ground piercing tools are used to install pipelines, power lines and information transmission cables such as fiber optics installed beneath the ground with a minimal amount of surface disruption. These tools include, as general components, a torpedo-shaped body having a tapered nose and an open rear end, an air supply hose that enters the rear of the tool and connects it to an air compressor, a piston or striker disposed for reciprocal movement within the tool, and an air distributing mechanism for causing the striker to move rapidly back and forth.
In the case of hard or rocky ground, it is often desirable to utilize pneumatic ground tools that incorporate movable bits or chisels at the tapered nose section of the tool to concentrate the striking force. For example, U.S. Pat. No. 6,273,201 to Randa et al., issued Aug. 14, 2001, the contents of which are incorporated herein for all purposes, discloses a reciprocating (front) head mole with a moveable chisel head that is axially independent of the remainder of the mole. Randa et al. facilitates transfer of striker energy directly to the leading end of the mole thereby improving productivity in hard ground.
In many cases, impact moles are started from pits dug in the earth. The mole is launched when the air valve supplying the mole with compressed air is opened, actuating the striker to begin impacting. The front end of the mole is forced against the sidewall of the launch pit until the mole penetrates the earth far enough so that sufficient friction force is produced between the mole body and the soil to hold the mole in position against the pneumatic reaction forces generated as the striker reciprocates.
Launching larger diameter pneumatic impact tools, for example in the range of 4″ diameter, tend to be considerably more difficult to start than smaller tools with diameters in the range of 2″. As the striker impacts the chisel and then the anvil, it generates a reaction force that first tends to move the movable head or chisel of the tool forward, then pull the tool body along behind. The striker then moves rearwardly in preparation for the next stroke. The difficulty arises as the striker reverses its direction and move forward for the next impact under the action of compressed air in the rear pressure chamber. The reaction force from this operation tends to move the tool body rearwardly. During normal operation when the mole is fully engaged in a borehole, friction between the surface of the tool body and the surrounding soil absorbs this reaction force, allowing the tool to make net forward progress through the ground. However, when the mole is first launched and only the head is engaged by the soil, the reaction forces generated by reciprocation of the striker can cause the movable head to lose engagement with the soil and requires the operator to manually apply an opposing force until the mole has penetrated the earth far enough so that friction between the mole and the soil holds the mole body in place. In soft soil, the friction between the mole body and the soil may not be sufficient to hold the mole in place, making start-up unusually difficult.
Most prior movable chisel-type ground piercing tools have used a metal spring or springs to bias the chisel in a rearward direction to return the chisel to its starting position after being impacted by the striker and partially absorb reaction forces during the forward stroke of the striker that would otherwise tend to make the tool body to move backward, especially during startup. For example, U.S. Pat. No. 5,095,998 to Hesse et al., issued Mar. 17, 1992, the entire contents of which are incorporated by reference herein for all purposes, discloses such an arrangement. However, the use of springs in this application raises issues of durability and design. Pneumatic impact moles normally operate at a relatively high impact frequency, typically in the range of 250 to 600 impacts per minute. Assuming an average travel rate of 1 foot/minute and 300 foot of boring per day, an impact mole may be subjected to 50 million impacts per year. Under these conditions, a spring is subject to fatigue fractures.